Porcellanite is a natural mineral raw material found in the earth in scarce quantities. It occurs in the coastal regions of different countries but also in rare dispersions in semi-closed basins such as Israel.
The porcellanite rock contains as basic and useful component, the so-called opal-CT, having the formula SiO.sub.2.nH.sub.2 O wherein n is between 0.1 and 0.3, which is a crystalline polymorf of SiO.sub.2 including H.sub.2 O in its network.
There were identified some special properties of this mineral matter which favorably differentiates it from quartz-sand with similar chemical composition (SiO.sub.2). First of all, this opaliform silica is highly reactive and thus can be easily dissolved in alkaline hydroxides (by hydrothermal process at low temperature and overpressure). In addition, this opaliform mineral is a cryptocrystalline form of silica (cristobalite or tridymite) with microscopic pores, large pore volumes and surface area, attributes which provide significant adsorptive capacity and related properties to this rare form cf silica.
A prevailing prerequisite of rendering profitable this useful mineral substance is the need to obtain the same in relatively high purity.
Exploration samples of the Israeli porcellanite occurrence indicate contents of opal (active silica) in the range of 20-65%. Accompanying mineral impurities are: limestone-calcite (CaCO.sub.3) amounting to 25-60%, flint (SiO.sub.2) and about 4-15%, and clays, limonitic iron oxides, quartz-sand, and other impurities as subordinate components (3-4%).
There are many potential uses, both direct and indirect, for opaliform active silica, however, all of them require a product purity of at least 85% and some require a purity of at least 90 or even 95%.
Thus some of the direct utilizations of opaliform active silica include:
a. Silica bricks (for open-hearth furnaces). PA1 b. Specialty glasses (for optical purposes). PA1 c. Laboratory glassware. PA1 d. Silica fibers (in precision instruments). PA1 e. Pyrogenic or fumed silica. PA1 f. Filler and reinforcing material (in rubber, paints, etc.). PA1 g. Sodium silica (water glass). PA1 a. Silica gel (catalyst, adsorbent filtering material, dehydrating agent). PA1 b. Detergent (in detergents and soap industry). PA1 c. Adhesive (adhesion to glass, wood, metal and paper). PA1 d. Binder (forming bricks or moulded objects). PA1 e. Zeolites and insoluble silicates (various uses). PA1 f. Oil recovery (enhancing oil flow in porous substances). PA1 a. Silica gels (96-99%) below this content have unsatisfactory surface area. PA1 b. Precipitated silica (95%) to achieve a minimum surface area of 40-45 m.sup.2 /g. PA1 c. Pyrogenic silica (97-99%.9%) to enable the manufacture of pyrogenic silica. PA1 d. Silica bricks (light weight aggregates) (92-95%) to obtain bulk density of 0.7-0.9 g/cm.sup.3. PA1 e. Oil recovery (88-95%) to ensure an oil absorption capacity of 1-3 g/g. PA1 f. Detergents (86-92%) to avoid deleterious effect of impurities (mainly CacO.sub.3).
Some of the e.g, indirect utilizations of sodium silicate include:
It is to be noted, however, that the projected prerequisite content of "active silica" for various applications are as follows: